Method for an Arrangement at a Paper Machine

ABSTRACT

The present invention relates to a method for reducing the spreading of additional agent(s) which is/are introduced into the process at a papermaking machine. A flowable paper pulp including fibres and a process liquid and having a consistency of not more than 10% is conducted towards the paper machine&#39;s headbox ( 5 ). Prior to the introduction ( 14 ) of said additional agent the consistency of the pulp is increased to a value higher than 10% by removing ( 8 ) process liquid and conducting ( 11 ) it in the upstream direction, while compensating process liquid is introduced ( 13 ) downstream. The present invention also relates to an arrangement at a paper machine, where separating means ( 8 ) are arranged in the process direction before means ( 14 ) for introducing additional agent (s). The arrangement further includes means ( 11 ) for conveying process liquid separated from the process in an upstream direction in the process, as well as means ( 13 ) for a downstream introduction of process liquid from a short circulation in order to compensate for the separated process liquid.

The present invention relates to a method at a paper machine forreducing the spreading of agents added to the process in a continuousprocess for preparation of pulp for a paper machine, whereby a flowablepaper pulp including fibres and process liquid and having a consistencyof not more than 10% is conducted to the headbox of the paper machine.

The present invention also relates to an arrangement at the pulppreparation for a paper machine in order to restrict a spreading ofadditional agents in the further process, whereby the process comprisesan inlet for a flowable pulp having a consistency of not more than 10%and including a fibre material and a process liquid, a closed cyclesystem including a headbox for the paper machine, as well as means foradding said additional agent.

In the present patent application a paper machine is defined as amachine for manufacturing paper as a continuous web. The expression pulppreparation as used in the present patent application indicates aprocess where starting from a pulp mass comprising fibres, usually acellulose material, and having a more or less firm consistency, a paperpulp mass is prepared under mixing to be transferred further to thehead-box, whereby the pulp mass includes not more than 10% fibrematerial, normally at least 1%, suitably 2 to 7% and usually 3 to 5 %,as well as added process liquid, usually water. Typically, such a pulppreparation includes a so called pulper where a fibre material from acellulose process is resolved during decomposition and agitation, andmixed with process water.

Traditional papermaking includes a process where a pulp mass whichessentially comprises a fibre material and process liquid is conductedvia a headbox for de-watering on a wire. A multitude of further stepsand arrangements are associated to the process, including vessels wherethe consistency typically is 3 to 5%, whereby much surplus liquid isconducted in an upstream direction back into the process.

In production of special quality paper such as decor paper, coloredpaper or other paper grades where valuable and perhaps process sensitiveadditional agent(s) such as titanium dioxide (TiO₂), pigment or the likeoften very expensive components are added to the process, the problemoccurs that the retention of these agents in the process is low, so thatthe so called white water typically can contain up to 1% or more of suchagents. In order to regain such agents a traditional process includes arecovering by means of, e.g., flotation in recovering devices, afterwhich the clear water from the flotation is recycled in the pulppreparation.

At the manufacture of such special quality paper it is usual that saidadditional agents are added to the process already during the pulppreparation, in order to obtain a sufficient mixing and contact timewith the actual papermaking pulp. In practice this means that the wholeprocess, in practice, will be dyed throughout. At a change of grade theprocess thus must initially achieve a balance, which results in a longbuilding-up time, while even a change of color requires an extensivecleaning of the system and results in high losses of material. Even assuch a colored paper, including pigment colored white paper, isdifficult to produce, and especially for decor paper the setting of theright color is difficult. Additionally there is the fact that a webbreak as such under these conditions lead to instability.

Since a decor paper, for example, will be impregnated at the furtherrefinement its optical properties will also change. The refractive indexfor the fibres and the impregnating agent are about the same, and thussuch factors as the concentration, distribution and optical propertiesof the pigment are critical for the quality of the product. For thisreason there is usually a need for making a test laminate prior to thestart of the actual production, and this takes about half an hour,during which time the pulp is run in a circulation on the machine.

As such the process can partially be made more effective by means ofsuch arrangements disclosed, i.a., in the applicant's internationalpatent applications No PCT/FI93/00214, PCT/FI94/00578, PCT/FI96/00052and PCT/FI01/00365, but some problems still remain, although saidarrangements make it possible to chose a suitable white water for eachspecific use. However, in a typical traditional process about 20 to 30kilograms of the additive titanium dioxide (TiO₂) for each 100 kilogramsof paper will pass the fibre recovery (see also Table I below). In atraditional fibre recovery plant using a flotation technique thechemicals will cause an agglomeration which reduces the optical effectof the pigments. Thus the titanium dioxide will lose in optical effect,and yet a part will be lost in the form of, for example, deposition inthe system. Additionally, there are, of course, the drawbacks caused byseparate arrangements for fibre recovery, costs of investments andoperating and costs depending on a contamination and cleaning of thesystems, as well as the more complicated process chemistry which arecovery in line with prior art requires.

Thus, the object of the present invention is to avoid the drawbackswhich until now have been connected to the use of pigments andespecially in connection with a change of the paper grade.

The present invention is based on the astonishing realization that byraising the consistency of the pulp at an early stage of the process itis, in practice, possible to cut the arrangement which in traditionalpapermaking is called the long circulation, i.e. a circulation systemwhere white water in accordance with traditional arrangements can runfrom the wire pit of the paper machine upstream in the system up to thepoint where pulp is added, for example to a pulper. By interrupting thiscirculation a flow of valuable additional agents directed upstream canbe prevented, for example a flow of titanium dioxide and other pigmentswhich otherwise partially would be lost and partially contaminate thewhole chain of process.

The characteristics of the present invention are disclosed in therespective characterizing portion of the appended independent claims,while the dependent claims disclose favorable embodiments of the presentinvention. Thus, the method according to the present invention ischaracterized in that the consistency of the pulp is increased, prior tothe introduction of said additional agent and suitably at a separateprocess stage, to a value of more than 10% by removing process liquid,in that this process liquid is conveyed upstream in the process, whilecompensating process liquid, which suitably can contain additionalagents, is added downstream in the direction of the process.

Correspondingly, a process according to the present invention ischaracterized in that separating means are arranged prior to the meansfor introducing the additional agent, seen in the direction of theprocess, but after said inlet, whereby these separating means arearranged to separate process liquid from the flowable pulp so that theconsistency of the pulp will increase to a value of more than 10%, insome cases favorably even up to 50%. The arrangement further includesmeans for conveying the liquid which has been separated from the processliquid in an upstream direction in the process, as well as means for adownstream introduction of process liquid from said circulation in orderto compensate the separated process liquid. This latter process liquidcan, as such, contain more or less additional agent(s)

Below the present invention is presented in more detail by reference tosome favorable embodiments and the appended drawings, where

FIG. 1 as a principle drawing discloses an arrangement according to oneembodiment of the present invention, and

FIG. 2 generally discloses a paper machine arrangement in accordancewith an especially favorable embodiment.

According to FIG. 1 a paper machine typically includes an inlet for afiber containing material, i.e. cellulose which usually is provided inthe form of dry sheets or, at so called integrated plants, directly froma cellulose plant in the form of a fibre containing process liquid. Ifthe incoming material is in the form of sheets the process starts with apulp preparation including a so called pulper 2 where the sheets aredisintegrated and mixed with process water, for example from a tank 3,to constitute a so called stock or papermaking pulp. The pulp passesthrough different process stages 4 a, 4 b, 4 c, 4 d, 4 e . . . which inthis context are essentially irrelevant, and finally it is spread viathe paper machine's headbox 5 onto a wire 6. The wire runs through amachinery 7 where the process liquid to an extensive portion isexhausted by suction in the form of so called white water, while themoist remaining pulp gradually by means of pressing 9 and drying isformed to paper.

Besides water the white water also to some degree contains chemicalsthat will do be retained in the paper. These chemicals follow with theprocess water which to some parts is used in the so called shortcirculation for dilution purposes at such process stages where saidchemicals are needed as such, or where they in any case cause only minorharm. White water used for other purposes is cleaned in differentprocesses and cleaning stages, on one hand to recover the chemicals and,on the other hand, to obtain a clean water which can be used in otherparts of the process. Each such cleaning process, is, however, to someextent incomplete and additionally requires both energy, time andapparatus.

Especially for special paper such as decor paper and other colored paperthe manufacturing volumes are relatively small. This makes theproduction of such grades fairly non-profitable in integrated papermills, and for this reason special grade paper is usually manufacturedin paper mills which receive the fiber material essentially in dry form,which leads to a need for process liquid already in the beginning of theprocess. In known processes this process liquid is, as a rule, taken inthe form of said long circulation from the cleaned white water. However,in practice also this cleaned process liquid contains a higher or lowerportion of impurities, i.e. mainly material which belongs to some otherpart of the process. For colored paper a part of these impurities areconstituted by, i.a., pigment such as titanium dioxide, which in orderto obtain a good mixing is usually added already in the pulper 1,whereby the system in practice will be fully dyed throughout. At achange to, e.g., a different color, this fact in combination with thetendency of these components, which are expensive as such, to deposit assediments in the different parts of the system leads, for example forcolored paper, in practice to a situation, where the whole system mustbe cleaned before the following quality can be manufactured.

The system according to the present invention differs essentially fromthe known technology described above. By momentarily raising theconsistency of the paper pulp by introducing separator means at least atone point of the process so that an additional separation 8, 8 a of theprocess liquids is achieved, which interrupts the long flows back up inthe system of such process liquid which is contaminated with valuableadditional agents. Earlier a separation has only been adopted in orderto prevent chemicals that disturb lower processes from flowingdownwards, while the idea is quite new that a prevention of valuableadditive agents from contaminating upstream also may be realized in acorresponding manner.

The principle behind the present invention is presented in Table I,where the material balance within different parts of the system isillustrated by means of the pulp consistency (in .), the volume(“X”=liters of liquid pro kilogram of pulp mass) and dry substance (inkilograms) for, on one hand, a typical process according to prior art(T) and, on the other hand, a process according to one embodiment of thepresent invention (U), where A indicates the values for the pulp massduring the initial stages 1 . . . 4 a of the process in accordance withprior art, A₁ indicates the values for the pulp mass after theseparation 8 in accordance with the present invention, B indicates thesituation at the headbox 5, C indicates the situation when the paper hasleft presses 9, D indicates “surplus water”, i.e., water that has to becleaned before it can be re-used, and E indicates the situation at theshort circulation:

TABLE I dry Consistency Volume substance % X kg Point T U T U T U A 4.025.0 100 A₁ 30 3.3 100 B 2.0 2.0 100.0 100 100 100 C 45 45 2.2 2.2 100100 D 1.0 0.5 22.8 1.1 22.8 0.6 E 2 100 200

From the above Table I it is evident that about 20 to 30 kilograms ofTiO₂ pro 100 kilograms of paper passes, i.a., a fibre recovery 10, whilethe volume of TiO₂-containing surplus water and its share of drysubstance will be radically reduced when the consistency of the pulpmass is momentarily increased in accordance with the present invention.

In practice, the removing separation of process liquid is favorablyeffected by means of such a thickening technique which is known per sefrom, for example, production of cellulose, bleaching and de-inking.Such arrangements are usually capable of increasing the consistency ofthe fibre containing pulp mass from an initial value of 2 to 10%, in thepresent case typically in the order of about 4 to 6%, to a final valueof even up to 50% and possibly even higher.

According to a favorable embodiment the separation is performed by meansof a pressing which favorably takes place prior to a beating 4 c. Thepressing at the thickening favorably then takes place in a screw or bandpress 8 a of a type known per se, so that the slushed pulp afterpressing obtains a dry matter content of more than 10%, especiallywithin an interval of 20 to 50%, suitably 25 to 40%, favorably at least30%.

The process liquid which is separated during the pressing is conveyed 11back in the upstream direction into the process, suitably back to thepulper 2. The liquid which has been separated from the process issuitably replaced 13 in a dispersing device 12 with an essentiallycorresponding amount of liquid taken from the circulation of the papermachine. Favorably this takes place in an arrangement according to theapplicants international patent application No PCT/FI99/00143, oralternatively by means of a screw arrangement or a mixer of some othertype. In order to obtain an optimal intermixing the dispersing device 12is favorably arranged in the process direction essentially directlybehind the separation 8, 8 a.

According to a favorable embodiment of the present invention the mainintroduction 14 of critical added agents such as very expensive pigmentsand coloring agents takes place in connection with the addition ofliquid to the dispersing device 12 (see FIG. 2), which provides aneffective intermixing of the additional agents in as early a stage aspossible after the concentration. FIG. 2 shows that additional agentsalso can be introduced 14 a, 14 b at later stages in the process. Sincethe introduction of at least a part of the additional agent(s) favorablytakes place at the same time as the introduction 13 of replacing liquidit is of little consequence if the replacing liquid is taken from such apart of the process where the residual content of additional agent(s)could be even high.

In practice, the above discussed arrangement where the consistency ofthe pulp mass is increased will lead to a situation where a removal andreturning of process liquid that functionally belongs to an earlierprocess stage in a way “makes room” for an introduction of suchadditional agent material one wants to prevent from spreading in theupstream direction. Thereby the latter additional agent material, andthus, e.g., the colored portion of the process, can be restricted to theshort circulation of the paper machine, while the long circulation, tothe extent that such a circuit possibly might be needed, is kept freefrom valuable and/or contaminating material.

Since at the especially favorable embodiment according to FIG. 2 thesystem is essentially self-cleaning, there is, in practice, a goodsupply of white water of different fractions, as well as a supply ofrelatively clean surplus liquid to the extent such liquid is needed inother portions of the system. Thus the correct clear fraction can bechosen for such needs, which further cuts down the losses of valuableadditional agent material. The minor amount of surplus water stillremaining can be treated in a fine filter to provide spray water etc,whereby the filter cake is returned to the dispersing device.

Thus, FIG. 2 discloses an especially favorable arrangement where theamount of water taken out from the system is diverted at a point wherethe concentration of material is as low as possible. According to thisembodiment this concentration can be reduced by arranging a suction box15 under the wire at such a point where a fibre web already has beenformed, so that the fibre web functions as a filter. This suction box 15is worked by a light vacuum whereby a careful filtration and a veryclean white water is achieved. This filtration reduces the amount ofmaterial so that this white water normally can be conducted 16 to anoverflow 17 without any need for any additional recovering.

By means of the arrangement according to the present invention a nearly100% retention of the valuable additional agents can be achieved, whileat the same time, e.g., the requirement for separate recovering devicesis eliminated, the no-load operation during the test laminatingprocedure is reduced and the building-up time after a change of color isshortened.

Above some favorable embodiments of the present invention have beendisclosed, but for the professional it is clear that the presentinvention also can be effected in many other ways within the scope ofthe appended claims.

1. A method for reducing the spreading of additive agent(s) introducedinto a a continuous process in pulp preparation for a paper machine,wherein a flowable paper pulp including fibres and process liquid andhaving a consistency of not more than 10% is conveyed towards a headboxof a paper machine, additive agent the consistency of the pulp isincreased to a value of more than 10% by removing process liquid andconveying this liquid upstream, while compensating process liquid isadded downstream.
 2. A method as defined in claim 1, wherein theconsistency of the pulp is increased so that the pulp corresponds to adry matter content of 20 to 50%, suitably about 25 to 40%, favorably atleast 30%, by separating process liquid from the pulp, suitably by meansof pressing, which process liquid is conveyed back upstream into theprocess.
 3. A method as defined in claim 1 wherein, prior to conveyingthe pulp forwards towards the headbox, a compensating liquid is added inthe form of additive agent(s) containing process liquid from the shortcirculation of the paper machine.
 4. A method as defined in claim 1,characterized in that the additive agent(s) is introduced in connectionwith a dispersing (12) of the pulp which has a increased consistency. 5A method as defined in claim 1, characterized in that said additiveagent(s) include(s) pigment or other color agents.
 6. An arrangement ata pulp preparation for a paper machine, for restricting a spreading ofadditive agent(s) upstream in the further process, where the processincludes an inlet for a flowable mass having a consistency of not morethan 10% and including a fibre material as well as process liquid, aclosed circulation system including a headbox for a wire for the papermachine as well as means for introducing said additive agent(s), whereinseparating means are arranged in the direction of the process ahead ofthe means for introducing the additive agent(s) but behind said inlet,which separating means are arranged to separate process liquid from theflowable mass so that the consistency of the mass increases to more than10%, means for conveying the process liquid which has been separatedfrom the mass upstream into the process, as well as means forintroducing process liquid from said closed system downstream in orderto compensate the separated process liquid.
 7. An arrangement as definedin claim 6, wherein said separating means are capable of increasing,suitably by means of a pressing, the consistency of the fibre containingmass from an initial value of 2 to 10%, typically in the order of about4 to 6%, to a final value of 20 to 50%, suitably 25 to 40%, favorably atleast 30%.
 8. An arrangement as defined in claim 6, characterized inthat means are arranged to add, in connection with or after said inletfor additive agent(s), compensating liquid from the short circulationsystem of the paper machine.
 9. An arrangement as defined in claim 6,wherein dispersing means are arranged to re-disperse, suitably underaddition of said compensating process liquid, the mass from which excessliquidhas been separated, whereby the means for introducing saidadditive agent(s) favorably are arranged in context with said dispersingmeans, suitably so that said dispersing means are arranged, in thedirection of the process, essentially immediately behind said separatingmeans.
 10. An arrangement as defined in claim 6, wherein a separatesuction box with a slight vacuum is arranged at such a portion of thewire where a fiber web already has been formed, and that means arearranged for conveying water from such a separate suction box a high upin the system towards a separate overflow.
 11. A method as defined inclaim 3, wherein said compensating process liquid is introduced inconnection with a dispersing of the pulp which has a increasedconsistency.